`
`0
`5,954,644
`[111 Patent Number:
`[19]
`United States Patent
`
`[Jettling et a1.
`[45} Date of Patent:
`Sep. 21, 1999
`
`USlKl'5954-644A
`
`|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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`[S41 METHOD FOR AMBIENT LIGHT
`SUBTRACTION [N A
`PHOTOPLETHYSMOGRAPHIC
`~
`‘
`1
`~
`1
`;
`MEASUREMENT m5TRUMLNT
`
`ll‘WCl‘llOl‘S! Allen Bottling. BI‘OOI‘I’llliJld, COIUJ Aifll'l.
`Martin. Sam Jose. (‘alll'.; Kurt
`Aronow, Lafayette, Colo.
`
`[73] Assignee:
`
`()hmeda lnc., Louisville, Colo.
`
`[21] Appl. No.: 0818235526
`
`5.152.29b
`5.385.144
`5,503,143
`3.5551882
`3.6322112
`
`11111992 Simons ................................. .. {£01492
`[£1995 Yarnunishi cl Ell.
`.
`123K333
`
`......... ..
`500.5323
`4.11906 Puloge [:1 al.
`11111336
`‘JII'J‘Jh Richardson {:1 :11.
`.............................. moms
`5.11997 Dial: £1 at.
`
`OTHER PUBLICATIONS
`
`"A New Family 01' Sensors for Pulse ()ximclry" by Kaslle,
`Noller, Falk. Bukta. Mayer
`and Miller Feb.
`1997
`Hewlclteliackanl Journal Amok; 7’
`Satin: el al.. "Microelectronic Circuits. 3rd Edilion." Saun-
`(Ich College Publishing, pp. 68 & 69, 1991.
`
`[55]
`
`References Cited
`‘
`‘
`y
`_
`H ‘ H
`1
`‘
`U-S- PAH—NI “(RUB/“45115
`REV 331-,“ W991 Isaacmn fl ai’ yyyyyyyyyyyyyyyyy '_ fimflm
`336322”
`“[972 swig). cl all
`I
`356;“
`3,802_.7?o
`411914 Tchang
`350141
`3.825.342
`T119714
`lubbers ct al.
`. 350141
`4H”..667
`1119?? Boher --------- A-
`323“
`4:316:54“
`531931 Hflmflg'fli
`“nil-{323
`
`
`
`Film“
`M3“ 249 1997
`Primary Examiner—Michael l’clIley
`l22
`HP
`_ & )
`Int. Cl." ........................................................ A613 5m 2mm”! fi*"mme’7_Br>’a_“[ [Ki WEE];
`[51|
`[52] U.S. C].
`......
`6110,1322; 600.3336
`I’m-"9’ 38“ or m"
`u m"
`u m’
`{ ww‘ ‘ "
`
`Field of Search .......................... .. 600E310. 322—328.
`[58]
`[5'1]
`ABS'I'RACT
`6001336, 340, 473, 4'16; 356139—41
`_
`_
`An Improved 1)holoplethysmographle measurement system
`is disclosed in which a portion of a time division multiplexed
`(TDM) signal represents an ambient light level. and other
`'I‘DM signal portions represent detected levels of two or
`more centered wavelengths of transmitted light. The ambiv
`em and detected light portions of the signal are simulta-
`neously applied to the inputs of an instrumentation
`amplilicds) so as to produce a continuous output voltage
`that is proportional to a difference in voltage between the
`ambient and dcieeted light portions of a TDM signal. Such
`an approach provides for ambient
`light
`level subtraction
`“"1 “1'4"” "0"“ and Comlmncmw'
`
`(most: Flower (:1 3|.
`{863,265
`911992 Uemalsu et al.
`5.144351
`5140503 More Kohno el al.
`
`v.
`
`5110322
`
`1281633
`
`422,321.15
`
`26 Claims, 3 Drawing Sheets
`
`AMPLIFICATION SECTlON
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`30
`
`\I-\
`
`SAMPLE AND HOLD
`
`
`
`CHA
`PRE-AMP
`
`
`
`
`
`LIGHT SOURCE
`DRIVER
`
`
`
`PROCESSINGi‘CONTROL
`SECTION
`
`50
`AMPLIFICA‘I’ION FILTERING
`
`DISPLAY
`
`0001
`
`US. Patent No. 8,652
`
`Apple
`APLl
`
`
`
`Apple Inc.
`APL1048
`U.S. Patent No. 8,652,040
`
`0001
`
`

`

`US. Patent
`
`Sep. 21, 1999
`
`Sheet 1 of 3
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`5,954,644
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`FIG.2
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`FIG.1
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`0002
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`0002
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`Sep. 21, 1999
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`5,954,644
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`US. Patent
`
`Sep.21,1999
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`5,954,644
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`

`
`
`1
`METHOD FOR AMBIENT LIGHT
`SUBTRACTION IN A
`I’HO’I‘OI’LI‘ITHYSMOGRAI’HIC
`MEASUREMENT INSTRUMENT
`
`FIELD OF THE INVENTION
`
`This invention relates to systems that utilize time division
`multiplexed (TDM) signals and, more particularly, to an
`improved photoplelhysrnog‘raphic measurement instrument
`in which an ambient light component is subtracted from a
`TDM signal. The invention is particularly apt for implemen-
`tat ion using instrumentation amplifiers.
`BACKGROUND OF THE INVENTION
`
`In the field of analog data transmission. one efficient data
`transmission technique is to utilize a TDM signal in which
`information corresponding with a plurality of sources is
`transmitted over a single data line. Data corresponding with
`each source is transmitted over the line in dedicated intervals
`which are generally regular in duration and sequenced. That
`is, at one particular point in time, data present on the line
`corresponds with only one of the sources. If the dedicated
`interval rate is sufficiently rapid, an apparency of continuous
`data transmission corresponding with each source is realized
`at the receiving end of the data line. In this regard, the TDM
`signal is de-multiplexed at the receiving end so as to separate
`the data into parallel channels, one corresponding with each
`source. De-muitiplexing is generally performed in a syn-
`chronous switching operation.
`In some systems, after de-multiplexing, a first series of
`signal conditioning steps is performed which operate on the
`parallel channel source data. Thereafter, a second series of
`steps is performed which, once again. require the signal to
`he in a TDM form. Re—multiplexing ofthc parallel channels
`is necessary to regain the TDM signal format required for
`the second series of steps. After the second series of signal
`conditioning steps, the signal is tle-mulliplexed a second
`time into parallel channels for completion of analog signal
`processing. The performance of each multiplexingx‘de-
`multiplexing iteration introduces switching noise into the
`resultant signal(s). As can be appreciated, such noise pre-
`sents system design considerations and limitations.
`Other limitations are also introduced by the performance
`of multiple de-multiplexinglmultiplexing iterations.
`Specifically. each time either of these operations is
`performed, the overall parts count in the system is increased.
`Such an increase may significantly limit the reliability of the
`system and increase manufacturing costs. Moreover,
`the
`associated increase in signal line length resulting from the
`additional parts, along with their
`interconnections, may
`serve to couple still further noise into the system from the
`ambient environment,
`thereby reducing system perfor-
`mance.
`
`The noted design consideralions/limitations are of par-
`ticular importance in medical
`instruments that determine
`pulse rate and blood oxygen saturation level via measure-
`ment of certain blood analytcs such as, for example, the
`concentration (as a percentage of total hemoglobin) of
`oxyhemoglobin (Ocllb), deoxyhcmoglobin (Rllb), carbony—
`hemoglobin (COHb) and methemoglobin (MetHb) of a
`patient. Such photoplethysmographic measurement instru-
`ments are configured to emit light of at least two dltfercnt,
`predetermined wavelengths through a selected portion of a
`patient’s anatomy (e.g., a finger tip}. The analytes to be
`identified within the patient '5 blood must each have unique
`light absorbance characteristics for at
`least
`two of the
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`
`5,954,644
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`2
`emitted wavelengths. By measuring changes in intensity of
`the transmitted (the light exiting an absorber is referred to as
`transmitted) light from the patient’s finger [or other suitable
`area of anatomy) at these wavelengths, each analyte may be
`determined. 'l'hcrcat'ter. characteristics such as blood oxygen
`saturation may be determined based on these analytes. Other
`characteristios such as pulse rate may be determined based
`on certain components of the transmitted light signal which
`passes through the patient '5 anatomy. Specifically, the trans—
`mitted light includes a large DC component and a smaller
`AC or pulsatilc component. By using the pulsatile
`component,
`the patient's pulse rate may be determined,
`since fluctuations in the pulsatile component are a function
`of arterioles pulsating with the patient‘s heart rate.
`In one photoplethysmographic measurement system
`known as a pulse oximeter. at least two wavelengths of light
`may be emitted during dedicated, alternating intervals. The
`transmitted light from the selected body portion is detected
`by a light-sensitive element (e.g., a photodiode). The light-
`sensitive clement then outputs a TDM signal that includes
`portions corresponding with each wavelength of the trans—
`mitted light. As will be appreciated, the photodiode is also
`sensitive to light which is present in the ambient environ-
`ment. Consequently. the 'I'DM output signal can include a
`corresponding ambient light component. Such component
`must be removed from the TDM signal for proper process
`ing. For this purpose, at least one interval within a TDM
`signal
`is typically dedicated to measuring a component
`corresponding with only the detected ambient light.
`For example in one known pulse oximeter, each emitted
`light
`level
`is immediately preceded by an ambient
`light
`interval which may also be referred to as a “dark time"
`interval. The system first tie-multiplexes the TDM signal
`into parallel channels. Signal pmccssing then proweds
`wherein a first series of steps performs preliminary filtering.
`Immediately following the first series of steps, the parallel
`channels are res-multiplexed. Next, a second series of steps
`is performed in which the re-mulliplexed signal facilitates
`subtraction of the dark time signal from the signal corre-
`sponding with each emitted light interval in a manner known
`in the art. Such subtraction process relies on a dark time
`interval immediately preceding each and every emitted light
`interval in a ‘I‘DM format. Following the second series of
`steps, in which ambient light subtraction is accomplished.
`the TDM signal is tie-multiplexed a second time into parallel
`channels prior to the completion of signal processing. Such
`multiple de-muiliplexingr’multiplexing raises the very noise
`introduction and cost concerns noted above.
`
`SUMMARY OF- THE INVENTION
`
`Accordingly, primary objectives of the present invention
`are to provide an improved photoplethysmographic mea-
`surement system wherein ambient light subtraction from a
`TDM signal is achieved with reduced noise andtor reduced
`componentry.
`in order to achieve such objectives, a system is provided
`having at least one TDM signal that includes at least a first
`identifiable portion that corresponds with detected light from
`at least one predetermined, light source plus any ambient
`light prcmnt in the system, and a second identifiable portion
`that corresponds with only the detected ambient light present
`in the system. In one aspect of the invention, the system
`further includes amplification means having first and second
`inputs and an output. The amplification means is configured
`to produce an amplified output on its output proportional to
`
`0005
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`

`

`5,954,644
`
`4
`length and the ambient light of interest. The first and second
`amplification means may advantageously comprise separate
`first and Second instrumentation amplifiers. each having a
`substantively linear response over a frequency range that
`accommodates the detected light. The first and second
`outputs from the amplification means are then processed to
`determincr'output certain characteristics including. but not
`limited to, a patient’s pulse rate and blood oxygen saturation
`level andfor specific blood analyte information such as. for
`example,
`the concentration (as a percentage of total
`hemoglobin) of oxyhcmoglobin (Ocl-Ib}, deoxyhcmogiobin
`(RIlh), carboxyhemoglobin (COl-lb) or methemoglohin
`(Moll-lb) or to otherwise provide an indication when one of
`such measures exceeds a predetermined level of interest.
`The concentrations of a plurality of the noted analytes of
`interest may be determined by using at
`least a common
`plurality of emitted wavelengths, provided that the analytes
`exhibit unique absorbence behavior at
`the emitted light
`wavelengths. By measuring changes in intensity of the
`transmitted light. for example. from a finger at the emitted
`wavelengths and based on the corresponding outputs of the
`amplification means. the aforementioned analytes are among
`those which may be determined in processing. Thereafter,
`characteristits such as blood oxygen saturation may be
`determined based on these analytes. Other characteristics
`such as pulse rate may be determined based on certain
`components of the transmitted light signal which passes
`through the patient‘s anatomy. Specifically, the transmitted
`light includes a large DC component and a smaller AC or
`pulsatile component. By using the pulsatile component. the
`patient’s pulse rate may be determined. since fluctuations in
`the pulsatile component are a function of arterioles pulsating
`with the patient’s heart rate.
`As will be appreciated, the present invention allows a
`system to be defined that employs only a single demuiti—
`plexing step within the overall system. In such a system, a
`TDM, such as described above. is demultiplexed into (i) a
`first signal corresponding to transmitted light at
`the first
`primary wavelength, (ii) a second output corresponding to
`the transmitted light at the second primary wavelength and
`(iii) an ambient light signal corresponding to the detected
`ambient
`light
`level. Thereafter, first and second ambient
`compensated outputs can be produoed by removing the
`ambient
`light
`level
`from the first and second signals,
`respectively, using the ambient light signal. The first and
`second ambient compensated outputs may then be separately
`conditioned and combinatively processed to determine one
`or more of the noted characteristics of interest.
`
`Switching noise can be reduced in the present invention
`since a TDM signal need only be demultiplexed a single
`time into individual channels.
`
`Additionally. pans count and complexity can be reduced.
`Finally, as noted. the invention is particularly apt for imple-
`mentations using instrumentation amplifiers, thereby further
`yielding improved system performance and reliability.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`
`
`3
`inputs. Means are provided for substantially simultaneously
`applying the first 'I'DM signal portion to the first input and
`for applying the second TDM signal portion to the second
`input. at substantially the same time, such that the amplified
`output produced by the amplification means is proportional
`to the dilference between the first and second signal
`portions, thereby achieving contemporaneous subtraction of
`the ambient light component and desired signal amplifica-
`tion. The contemporaneous amplification and ambient light
`level removal may be advantageously performed using an
`instrumentation amplifier.
`In another aspect of the invention. the system is config-
`ured for emitting light through a region of interest at two- or
`more different primary wavelengths in an environment
`which includes an ambient light level. The system separately
`detects the ambient light level and transmitted light level for
`each primary wavelength that has passed through the region
`of interest, such that the levels of detected ambient light and
`transmitted light form corresponding portions of a TDM
`signal. The level of transmitted light detected at each pri-
`mary wavelength includes the ambient light level. To pre-
`pare for removing the ambient light level from such tra us—
`mitted light
`levels. demultiplexing means is provided for
`demultiplexing the TDM signal to provide (i) a first portion
`signal corresponding to the transmitted light level at the first
`primary wavelength. (ii) a second signal portion correspond-
`ing to the transmitted light
`level at
`the second primary
`wavelength and (iii) an ambient light signal portion corre-
`sponding to the detected ambient light level. A first subtrac-
`tion means then produces a first ambient compensated
`output corresponding to the first primary wavelength by
`removing the ambient light level from the first signal por-
`tion. Similarly, while second subtraction means. separate
`from the first subtraction means, produces a second ambient
`compensated output corresponding to the second primary
`wavelength by removing the ambient light level from the
`second signal portion. Such separate first and second sub-
`traction means may, for example, comprise first and second
`instrumentation amplifiers. A processor means is employed
`to determine the value of the characteristic(s) of interest
`within the region of interest based on the first and second
`ambient compensated outputs.
`In a primary embodiment of the invention, a photopl-
`ethysmographic measurement system includes means for
`emitting light through a portion of a patient’s anatomy at two
`or more dill'ercnt, predetermined and centered wavelengths
`(cg. by intermittent emission}. The transmitted portions of
`the emitted light for each centered wavelength and the
`ambient light
`level are detected so as to form respective
`ambient and detected light signal portions within the TDM
`signal. First and second amplification means are provided,
`each of which includes a first input. a second input and an
`output for producing an amplified output. The output pro-
`duced by each amplification means is proportional
`to a
`dilference between signals present on its first and second
`inputs multiplied by a predetermined and variable gain. The
`system is configured to apply the ambient light signal to the
`first input of each amplification means while, at substantially
`the same time, applying the transmitted detected light sig-
`nals to the second input ofthe first and second amplification
`means. Consequently, the first amplification means produces
`a first output that is proportional to the difi'ercnee between
`the detected signal corresponding to a first predetermined,
`centered wavelength and the ambient light level; and the
`Second amplification means produces a second output that is
`proportional to the diflerence between the detected signal
`corresponding to a second predetermined. centered wave-
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`at the preamp output by the system of the present invention.
`
`For a more complete understanding of the present inven-
`tion and further advantages thereof, reference is now made
`to the following Detailed Description, taken in conjunction
`with the Drawings, in which:
`FIG. 1 is a diagrammatic illustration of a photoplethys~
`mographic measurement system implementing the present
`invention.
`
`FIG. 2 is a waveform illustrating a TDM signal produced
`
`0006
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`

`

`
`
`5
`FIG. 3 is a block diagram illustrating the components
`which make up selected portions of the system of FIG. 1.
`FIG. 4 is a schematic representation of an amplification
`section used in the system of FIGS. 1 and 3.
`DE‘I‘AILED DESCRIP’I‘ION 01'? ‘l‘I-IE
`INVENTION
`
`FIG. 1 is a diagrammatic illustration of a photoplcthys—
`mographic measurement system embodiment, generally
`indicated by reference numeral 10, constructed in accor-
`dance with the present invention. As will be described, the
`embodiment utilizes a time division multiplexed ('I'DM)
`signal in conjunction with an instrumentation-type amplifier.
`The system is configured to apply specific portions of the
`TDM signal to the inputs of the instrumentation amplifiers
`so as to produce continuous output voltages that are pro-
`portional to differences in voltage between dill'erent portions
`of the IBM signal.
`System ill
`includes a Sensor probe 12 and a signal
`conditioning’processing assembly 30 mounted in housing
`14. Probe 12 is configured for emitting light 16 Centered
`about a first wavelength and light 18 centered about a second
`wavelength. Light of the first and second wavelengths is
`alternately emitted at regular intervals from Iirst and second
`light sources 20 and 22,
`respectively, which may,
`for
`example. comprise light emitting diodes or laser diodes. One
`known combination of first and second wavelengths com-
`prises light centered about 660 nm and 940 nm, respectively.
`It is to be understood, of course, that many other combiner
`tions can be employed. Furthermore. it should be appreci-
`ated that the present invention can be employed in systems
`utilizing light of more than two centered, or primary, wave-
`lengths of light.
`Continuing to refer to FIG. I, a portion ofthe emitted light
`is transmitted through a portion ol'a patient’s anatomy, such
`as a finger 24, and is detected by a lightvsensitive device. In
`the described embodiment, a photodiode 26 is utilized.
`Other areas of the patient’s anatomy may also be used
`provided that the transmitted light suitably passes through
`such areas. In this regard, the output indications provided by
`system 10 pertain to arterial blood flow data. More
`particularly. based upon the absorption of light at the emitted
`wavelengths certain characteristics may be determined
`including, but not limited to, a patient's pulse rate and blood
`oxygen saturation level, including the concentration (as a
`percentage of total hemoglobin) ol' oxyhemoglobin (Uzi-lb),
`deoxyhcmoglcbin {RI-lb), carboxyhemoglobin (COllb) or
`methemoglobin {MetI-Ib).
`Sensor probe 12 is electrically connected to the signal
`conditioninglprocessing assembly 30 via Inulti-corlductot'
`cable 32. A first set 34 of conductors within cable 32 carries
`drive signals to light sources 20 and 22, while a second set
`36 of conductors is used to bias photodiode 26 and to carry
`a 'I'DM signal to the signal conditioningi’processing assem-
`bly 30.
`Referring to FIG. 2 in conjunction with FIG. 1, the TIJM
`signal 38 includes a series of pulse groups 40 output by the
`photodiode 26 in response to the detection of light passed
`through finger 24. Each pulse group includes, in this case, a
`negative going “light 1" {hereinafter "LTl") portion, or
`interval, and a negative going "light 2" (hereinafter “L‘l‘2“)
`portion. or interval, corresponding to the detected levels of
`light at each of the two transmitted wavelengths. Ambient
`light is also detected by photodiode 26 together with the
`detected light corresponding with the light at
`the lirst
`wavelength 16 and light of the second wavelength 18. This
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`means of control lines 59. Synchronous control of switch 56
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`5,954,644
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`6
`ambient light is manifested within the output signal of the
`photodiode 26 as an ufl‘set voltage. That is, the [It and LTZ
`portions each include an ofi'set which results from ambient
`light that is incident upon the photodiode 26 during the time
`that the LTl and LT2 signal portions are generated. In order
`to facilitate removal of the offset. 'I'DM signal 38 includes a
`"dark 1"{hcreinat’ter "I)KI”) portion, or interval, immedi-
`ately preceding Eli. and a “dark 2”(hercinafter “DK2”)
`interval 48 immediately preceding L'I'Z. The voltage level
`during each of the DKl and DK2 intervals represents the
`ambient
`light
`level
`incident upon photodiode 26 in the
`absence of transmitted light at the first wavelength 16 or
`transmitted light at the second wavelength 18.
`By way of example and in total darkness, a signal
`corresponding to VJ”, is output by the photodiode which
`may be offset slightly from the Zero voltage level Vnw. The
`difference in voltage between V3,", and DKl, and between
`V3", and DK2, illustrated as Vamb, represents the overall
`ambient
`light offset present
`in TDM signal 38. Such an
`ambient
`light
`level may result
`from any light source
`including, for example. room lighting or sunlight. As will be
`appreciated. subtraction of the DKl and 0K2 voltages from
`the Eli and LT2 portions, respectively, will result in elimi—
`nation of both the ambient light data and the photodiodc dark
`current from the signal of interest, i.e., data corresponding
`with the transmitted light 16 and 18 which has passed
`through finger 24. It should also be appreciated that ambient
`light may not produce a D.C. offset as illustrated in FIG. 2.
`For example, certain common types of lighting induce a
`time-varying AC. ofiset. One such type of lighting is 120
`Volt A.C., 60 Ha. powered fluorescent lighting which may
`produce ambient
`light pulses at a frequency of 120 Hz.
`Therefore, circuitry for providing ambient light level sub-
`traction should also be ell‘ective in the removal of such
`
`time-varying ofi‘set signals, as will be described in further
`detail below.
`
`’l'DM signal 38 may be
`is to be understood that
`It
`configured in a number of different ways in accordance with
`the present
`invention.
`in the described embodiment, each
`pulse group 40 includes a dark time interval preceding each
`light time interval. It is noted that some prior art systems
`require this signal configuration to perform ambient
`light
`subtraction. The present invention also contemplates arnbi»
`ent light subtraction utilizing one dark time interval per each
`pulse group 40, as will be described further below.
`Referring to FIGS.
`1
`through 3, signal conditioning!
`processing assembly 30 will now be described. A
`processingfcontrol section 50 is included which provides
`drive signals to a light source driver section 52 through a
`control signal line 53. The light source driver section may be
`configured, for example, to drive LEDs, laser diodes or other
`such suitable light sources which may become available.
`The light source driver section 52 provides drive signal
`waveforms to probe 12 so as to excite sources 20 and 22 to
`emit light 16 (of the first wavelength] and light 18 {of the
`second wavelength). In turn, pholodiodc 26 detects the light
`passing through the selected body portion to output the TDM
`signal 38 of FIG. 2.
`As shown in FIG. 3, TDM signal 38 is coupled to a
`preamp section 54 via conductors 36 from probe 12. Preamp
`54 converts the relatively small magnitude current of TDM
`signal 38 to a voltage level more useful for processing. A
`silicon switch 56 is connected to preamp 54 through resistor
`58 (c.g., a 2.1 KW resistor). In the present example, silicon
`switch 56 compriSes a single pole, quadruple throw switch
`which is controlled by prttccssing’control section 50 by
`
`0007
`
`

`

`
`
`7
`is coordinated by processingfcontrol section 50 with drive
`signals provided to light source driver section 52 such that
`TDM signal 38 is de-multiplexed. Specifically. silicon
`switch 56 outputs four data channels A, B, C and D wherein
`channel A comprises the LTl signal portion. channel B
`comprises the URI signal portion. channel C comprises the
`L]? signal portion and channel D comprises the IJK2 signal
`portion.
`Ii'oilowirtg de-multiplexing, the signal on each channel,
`A—D, charges one of four holding capacitors 60n—d(c.g., 1.0
`,ttF capacitors). These holding capacitors are configured with
`resistor 58 to form part ol~ a sample and hold circuit (as well
`as a low-pass filter) in which an average value of each
`channel is stored [or that cycle.
`Thereafter, signals on each of channels A—D are filtered
`by one of [our first order low-pass filters 62n—a'. Each filter
`includes a resistor 64 (e.g., 60.4 KB resistors) and a capaci-
`tor 66 (e.g., {Ll
`.trF capacitors).
`In accordance with the
`present
`invention,
`the sample and holdflmv pass circuit
`comprised of resistor 58, capacitors 60 and silicon switch 56
`cooperates with low-pass filters 62 so as to simultaneously
`and continuously apply signals L't‘t'. DKt', LT." and DK.’
`to an amplification section 68. It should be appreciated that
`the values of resistor 58 and resistors 62.
`in the low pass
`filter, are selected along with the values ot‘ capacitors 60 and *
`capacitors 62. in the low pass filter, to filter out common,
`time-varying offset voltages such as those produced by
`fluorescent lighting to effectively remove the time-varying
`ambient light signal component. The values of these various
`passive components may be modified as required by the
`TDM signal being processed and. in fact, the components
`may he of different values from one channel to the next for
`a particular application. For example, the circuitry of FIG. 3
`may readily be modified for a ’l‘DM signal which includes a
`single dark time interval per pulse group (not shown). In
`such a case for a two-wavelength system, channel A may
`process the LT] signal, channel B may process the LT2
`signal and channel C may process a DK signal, with channel
`D not being required.
`Continuing to refer to FIG. 3, amplification section 68
`includes first and second instrumentation amplifiers 70a and
`70!). As employed herein, the term “instrumentation ampli-
`fier" refers to an amplifier which has an output. V,,M=[(V,_—
`V_) G+V,,.,] where V, and V_ are the inverting and non-
`inverting inputs. respectively. V”), is a reference voltage
`which may be set to ground. (3 is the gain. Typically, the
`common mode rejection ratio, CMRR, is very high. for
`example, greater than 100 dB. The instrumentation amplifier
`may be a single integrated circuit or made up of a group of
`integrated circuits andt’or discrete transistors.
`A plurality of control lines 72 connect processingfcontrol
`section 50 with amplification SCCllOI'I 68. Each amplifier 70a,
`70!) includes an inverting input, indicated by a minus sign,
`and a non-inverting input, indicated by a plus sign. I.Tl' and
`DKt' are applied to the inverting and non-inverting inputs of
`amplifier 70a, respectively, while I.‘ 2' and DIQ‘ are applied
`to the inverting and non-inverting inputs of amplifier 70!),
`respectively. Alternatively, where a single dark time interval
`is preSenled within each pulse group, the dark time channel
`is applied to both of the non-inverting inputs of amplifiers
`700 and 70b.
`
`if]
`
`15
`
`3E)
`
`35
`
`40
`
`45
`
`50
`
`55
`
`6t!
`
`The gain, G. of each amplifier 70a, 70b is adjusted by
`varying the resistance between a pair of terminals.
`Specifically, a first variable resistor 74 is connected between
`terminals 76 and 78 of amplifier 70o, while a second
`variable resistor 80 is connected between terminals 82 and
`
`()5
`
`0008
`
`5,954,644
`
`8
`84 of amplifier 7%. Variable resistors 74 and 80 are adjusted
`by processingr’control section 50 using control line sets 72a
`and 72!), respectively. Amplifier 70:? includes an output 86
`and is configured to output a voltage according to the
`difference in voltage level between its inverting and non-
`inverting inputs multiplied by the gain of the amplifier, as
`determined by the setting of the variable resistor. Amplifier
`70b includes an output 88 and is configured in the manner
`of amplifier 700. Amplifiers 700‘ and 70!) each include a
`reference input 90 and 92, respectively. The reference inputs
`may be grounded but, alternatively, they may he provided
`with ofl’set voltages V0”, and Von—h on control lines 72c
`and 72d, respectively, which are added to each amplifier’s
`output voltage. Thus, each amplifier outputs a voltage
`VAMP" or VAMl’b, respectively,
`in accordance with the
`equations:
`v.4 1 tPa‘v{)FJ‘nt-+l C'AINH x U—Tl ‘DKl Jli 0’
`VA! we‘vofi'fll GMNu" (Lila—D K2 Jl
`
`wherein V059, and Vac”, are provided from the processing{
`control section and wherein GAIN“ and GAIN” are,
`likewise, determined by the processingtcontrol section and
`implemented via settings of variable resistors 74 and 80.
`VAMP" 86 and VAMH, 88 are provided to an amplificationt
`liltering section 91, then to processingfcontrol section 50.
`Following processing, data is provided by prooessing’
`control section 50 to a display 92 including a display screen
`94 ot' a suitable configuration including, for example, LCD
`and CRT types. Information and related warnings are pro-
`vided in conjunction with or as an alternative to visual
`display. For example, in the event that the determined value
`of a monitored characteristic falls above andfor below
`predetermined threshold values an audio alarm may sound to
`alert attending medical personnel.
`One advantage in the configuration of the circuitry of FIG.
`3 relates to control of the gain and offset voltage settings of
`amplifiers 70 by the processinglcontrol section. Specifically,
`to achieve a relatively high signal to noise ratio, the gain and
`offset settings provided by processinglcontrol section 50
`should cooperatively center each amplil'ier’s output within
`the input
`range of the pmcessirtgfcontrol measurement
`system, at
`the same time, maximizing the swing of the
`output voltage therebetween without clipping either the top
`or bottom of the waveform. I’rocessingrcontrol section 50
`may accomplish this task by monitoring the VAM,,,,. and
`mer signals received from amplificationi’filtering section
`91, or “my” and VAMPb, received directly from amplifica-
`tion section 68 using, for example, automatic gain control
`techniques which have previously been implemented in the
`art using, for example, control algorithms.
`FIG. 4 illustrates an amplification section 68. In that all of
`the instrumentation amplifiers which form part of system 10
`are arranged in a directly analogous manner to amplifier 7%,
`only instrumentation amplifier 70a and its associated cir—
`cuitry including channels A and B for producing VAN“, will
`be described in detail. De-multiplexed signals LT] and DKI
`of channelsA and B are first applied to capacitors 6|]n and
`60!), respectively, which form the sample and hold circuit in
`conjunction with resistor 58 (s